Fiber Bragg Grating (FBG) sensors are used in a variety of applications ranging from damage detection in composites to dynamic structural strain monitoring to long-term strain monitoring in the construction industry. FBGs are also used as temperature sensors, pressure sensors, etc., with a very high measurement accuracy. Despite being highly sensitive and accurate, the applications of FBG sensors are limited. The most genuine problem that FBGs face is the bulkiness and cost of its interrogation system, which is the optical spectrum analyzer (OSA). These problems limit the application areas of FBGs.
Several FBG interrogation designs have been proposed which rule out OSA, but they are only partially promising. A matched-filter interrogation has been demonstrated for strain measurement. Identical gratings are used as notch filters in this system. These notch filters are mounted on small stretching devices driven by piezoelectric (PZ) stacks which make this technique complicated and limits the strain measurement range to ±200με. Mechanical strain amplification is needed to expand the measurement range, which makes the system even more complicated. An FBG demodulation system utilizing tunable Fabry-Perot wavelength filter has also been developed. Again, a piezoelectric transducer is used to adjust the cavity spacing in the Fabry-Perot wavelength filter. It makes the system complicated and dependent on the performance of an electrical component, like a piezoelectric transducer. Further, a passive wavelength demodulation system has been demonstrated which uses a commercial infrared high pass filter. The resolution of this system is very poor (around 400με). One other known system employs an asymmetric grating as a wavelength-to-amplitude converter for linear sensing structures. The asymmetric grating employed in this technique is difficult to fabricate. In another low-cost FBG interrogation system, a long period grating (LPG) has been used as an edge filter converting strain-induced wavelength variation into optical power measurement. The LPGs are extremely sensitive to external perturbations such as temperature, strain, etc., which makes this interrogation system unsuitable for external applications. Moreover, the LPGs are known for their very high sensitivity to the refractive index of the surrounding medium.
There is also an FBG demodulation method using UV-induced birefringence of the optical fiber. To interrogate the wavelength shift in the FBG, the demodulator uses the wavelength-dependent travel-length of the reflected light from a chirped fiber grating. This method requires a few other expensive optical components and the range of this demodulator is very limited (only up to 3000με). A multiplexed Bragg grating sensor configuration utilizing chirped FBG as interrogator is also known. This design is complicated and expensive as it employs Erbium-doped fiber amplifier, RF generator, phase detector etc. In another complex FBG interrogation technique, there is provided a chirped fiber grating based Sagnac loop. Though the claimed resolution is good (around ±5με), the strain measurement range is very limited (around ±250με). An interrogation technique using identical chirped FBGs has also been proposed for strain sensing with a resolution of 5με. In this technique, the strain measurement range could be as high as 10000με, but it can only measure the strain in one direction (tension or compression), which limits the application of this design.